In the printing arts, rotary machines are used for printing or otherwise applying ink or other liquidous compositions to travelling substrates, including sheets or webs. Such machines conventionally comprise a frame and a rotary receiving roll carried by the frame. Such rolls conventionally have a longitudinal rotational axis and a peripheral surface.
In some instances such receiving rolls act as printing rolls, having indicia formed on their peripheral surfaces for forming a graphic image on the substrate. In other instances, the receiving rolls act as inking, fluid metering or transfer rolls. More particularly, ink is applied to the receiving roll and then transferred from that roll directly or through an intermediate roll or rolls to a printing roll. A frequently used roll of this type is the "anilox" roll, the peripheral surface of which has a variety of tiny, closely spaced, shallow depressions called "cells", and these cells retain ink or other liquidous composition supplied thereto by any appropriate delivering means, such as a bath, fountain or the like.
Doctor blades are used to level and/or remove excess composition from the peripheral surfaces of receiving rolls, including those receiving rolls which act as printing rolls, and those which act as inking, metering or transfer rolls. A doctor blade has a doctoring edge which contacts the peripheral surface of the roll along a line of contact which is usually at least generally parallel to the rotational axis of the roll.
Frequently a doctor blade will be canted, so that one of its surfaces is at an acute angle to a plane which is tangent to the roll peripheral surface along the contact line. If the doctor blade is a flat member or a thin wedge, which is often the case, the other surface of the doctor blade will generally be at an obtuse angle to the above-mentioned plane. When the peripheral surface of the roll approaches the contact line from the acute angle side, its action may be said to resemble wiping. When the surface of the roll approaches the contact line from the obtuse angle side, the action of the doctor blade may be described as scraping, and a blade so mounted can be referred to as a "reverse angle doctor blade". Reverse angle doctor blades may be used alone or in combination with doctor blades disposed in the opposite direction.
One of the most common problems encountered with reverse angle doctor blades is blade chatter, which can be caused by machine vibration, excessive extension of the blades from their holders and mal-adjustment. Thus, numerous precision adjustments must be made by the operator in the attempt to minimize blade chatter and such adjustment is not always sufficient to fully solve the problem.
Another challenge encountered with doctor blade systems is the problem of containment of fluids and fume emissions within the doctoring system. It is possible to provide closed doctoring systems in which both normal and reverse angle doctor blades are applied to the same receiving roll and in which the intervening space between the doctor blades is fully enclosed with the ink supply being maintained between them. While this minimizes the potential for escape of toxic fumes into the workplace surrounding the equipment, the enclosure can also complicate the process of making the numerous and delicate adjustments required to eliminate blade chatter.
Clearly, a need remains for improvements to ameliorate these difficulties. It is an object of the present invention to meet this need.